Explosive Cyclogenesis and Large-Scale Circulation Changes: Implications for Atmospheric Blocking

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Abstract
Large-scale circulation changes attending explosive surface cyclogenesis are quantitatively examined in two cases selected from recent winter seasons. Both cases feature a rapidly deepening surface cyclone over the western Atlantic Ocean, but changes in the 500 mb geopotential height field near the cyclone differ in each case. One event, during January 1977, is characterized by the retrogression of an anticyclonic vortex (blocking high) in the 500 mb height field downstream of the surface cyclone. The second case, in February 1978, is distinguished by the formation of a 500 mb cyclonic vortex (cutoff low) upstream of the surface cyclone, but no downstream anticyclonic vortex is observed. The retrogression of the January 1977 block over the Atlantic Ocean coincides with the migration of a 500 mb synoptic-scale perturbation (associated with the surface cyclone) from a planetary- scale trough over North America toward a planetary-scale ridge over Europe. In the February 1978 case, the blocking cyclo... Abstract Large-scale circulation changes attending explosive surface cyclogenesis are quantitatively examined in two cases selected from recent winter seasons. Both cases feature a rapidly deepening surface cyclone over the western Atlantic Ocean, but changes in the 500 mb geopotential height field near the cyclone differ in each case. One event, during January 1977, is characterized by the retrogression of an anticyclonic vortex (blocking high) in the 500 mb height field downstream of the surface cyclone. The second case, in February 1978, is distinguished by the formation of a 500 mb cyclonic vortex (cutoff low) upstream of the surface cyclone, but no downstream anticyclonic vortex is observed. The retrogression of the January 1977 block over the Atlantic Ocean coincides with the migration of a 500 mb synoptic-scale perturbation (associated with the surface cyclone) from a planetary- scale trough over North America toward a planetary-scale ridge over Europe. In the February 1978 case, the blocking cyclo...

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